Machine Design

Process Considerations

Engineers must not only consider design and material requirements, but also should take the actual welding process into account. Part positioning and welding procedures affect weld quality. Designers must compensate for distortions induced by welding. And once the parts are welded, they must be inspected. The quality of welds is ultimately the designer's responsibility.

Use jigs, fixtures, and positioners: Jigs and fixtures hold parts in correct alignment for welding. Fabrication shops may have suitable ones available, but when designers and manufacturing engineers have to specify new ones, this cost must be included in the cost of welding.

Motorized positioning equipment can turn massive components to position them for welding. If such equipment is available, workers must mount the components on the equipment and make sure that it is properly aligned before welding. The time this takes must be compared to the welding time involved. If mounting time equals or exceeds the welding time saved, it is better to choose a welding process that will operate in the position the structure is already in.

Welding position is important. Thoughtful designers can arrange to have most welding done flat, so processes such as submerged-arc welding can be used. Flat-position welding is easier for welders, yields better quality, and improves production rates. Other positions limit the processes that can be used. The lack of access also makes inspection more difficult, and both quality and productivity are likely to suffer.

Specify welding procedures: Ideal welding procedures are those that produce acceptable-quality welds at the lowest overall cost. The procedures should be tailored to the application and should spell out exactly what welds are to be made and how they are to be made.

Procedure variables include the welding process, conditions, and operation details; joint details; filler metal and shielding gas (if any), and safety.

Different procedures produce different weld qualities, such as strength only, commercial, or code quality. Strength-only procedures produce the highest welding speeds at the lowest costs. Appearance and quality are not important in these applications. Defects and imperfections are acceptable as long as the welds perform satisfactorily under service conditions. Tests should be specified to confirm performance of strength-only welds.

Commercial-quality procedures produce higher quality levels and appearance. Such welds are crack free and pressure tight. They look good and meet joint strength requirements. However, commercial-quality welds may have minor defects.

Code-quality procedures produce welds of the highest quality and best appearance. The welds are made under carefully controlled conditions and therefore are relatively costly to make. Commonly used codes include AWS Structural, AISC Buildings and Bridges, ASME Boilers and Pressure Vessels, and AASHTO Bridges. Code-quality welds must be free of defects and pass nondestructive testing. In practice, this usually means welds that are free of cracks and have limited porosity, undercut, or other flaws.

Control and correct distortion: During welding, localized heating causes temperature variations, which may cause residual stress and permanent distortion. The designer must control and correct distortion. One of the best techniques to lessen distortion is restrained assembly, in which jigs or fixtures restrain the parts rigidly so they cannot move. This reduces distortion, but stress may build up within the joints and may increase the chance of cracking.

Thin materials tend to distort more, so they should be restrained heavily. For example, clamp down 1/16-in. sheets as much as possible so they do not warp and buckle. They still will move when released from the jig, but less than if they were welded without restraint.

On thicker plates, welding sequencing controls distortion more effectively. Materials are allowed to move one way with one weld and to move back with the next. This movement minimizes distortion, but residual stresses in these joints still may be high.

Designers should also try to design parts so welds are made near the part's neutral axis, because weld-shrinkage forces away from that axis may cause angular distortion. Furthermore, if two equal-sized welds are made, they should be equidistant from the neutral axis, and done simultaneously whenever possible. Also, welds far from the neutral axis should be made smaller.

The presetting assembly method can also reduce distortion. Segments of a structure are prebent opposite to the predicted distortion to counter distortion in the complete joint. Weld shrinkage brings the segments to the desired position.

Distortion also can be reduced by avoiding overwelding, as well as by using subassemblies.

Specify heat treatment: Stress in most welded materials can be relieved by heating them while restraining them in the desired position. The weld materials relax, relieving some of the residual stress. However, the structures must be heated uniformly, and cooling must be slow so that stresses do not redevelop.

Stresses in large structures are not easily relieved through heat treatment. Local stress relief is sometimes possible but may be impractical due to the large heat sink afforded by these structures.

Stress relief is critical for dimensional stability in parts that will be machined. If structures contain much residual stress, they will move and distort when the machining process begins removing material.

Consider cleaning and inspection: Joints should be accessible not only for welding, but also for cleaning and inspection. Cleaning may vary from removing the slag covering to profiling the weld contour by grinding or machining.

Certain joints are inherently difficult to inspect. Design critical joints so they can be inspected with confidence. For example, fillet welds and spot welds are difficult to inspect. Full-penetration, butt-joint welds are much easier to inspect by standard methods.

Designers also must know something about inspection to enable inspectors to get at welds. Inspectors first examine welds visually. Surface discontinuities give useful information about workmanship, often related to the total quality of welds. Shoddy-looking welds probably contain internal discontinuities. On the other hand, beautiful welds can conceal serious internal flaws. Thus, visual inspection often is followed by more revealing inspection techniques, such as ultrasonic or radiographic testing.

TAGS: Fasteners
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